The present invention relates to a method of manufacturing a reinforced plastics material part by using a drapable sheet and a mold.
The invention relates more particularly but not exclusively to manufacturing a structural part for a motor vehicle.
It is known to use drapable sheets constituted by woven fabric made of polypropylene and glass fiber yarn. An example is the material sold under the trade name TWINTEX by Vetrotex.
There exists a need to reduce cycle time in the manufacture of reinforced plastics material-parts so as to obtain higher rates of throughput.
The invention achieves this by a novel method of manufacturing a reinforced plastics material part from-a sheet of drapable material by using a mold comprising first and second mold portions that are movable relative to each other, the method being characterized by the fact that it comprises the following steps:
placing the sheet on the first portion of the open mold;
pressing the sheet against the first portion at one or more locations by means of one or more pushers mounted on the second portion of the mold and movable relative to said second portion;
closing the mold and compacting the sheet; and
unmolding the part.
By means of the invention, the drapable sheet is preformed on the mold cavity in a manner that is relatively simple and fast since the pushers used for this purpose are mounted directly on the second mold portion.
This avoids the need to use handling means external to the mold for preforming the sheet prior to the compacting step.
Advantageously, the sheet is preheated prior to being placed on the first mold portion.
The sheet is advantageously pressed against the first mold portion initially substantially in the center thereof so as to make the sheet easier to shape.
The sheet can thus be pressed against the first portion of the mold by means of one or more pushers situated substantially in the center of the mold, and then by means of other pushers, with the pushers that are furthest from the center of the mold being actuated last.
This makes it easier to cause the sheet to follow the outline of the mold cavity.
In addition, it reduces the risk of unwanted folds being formed.
Advantageously, the portion of the sheet which projects beyond the join plane after the mold has been closed is trimmed off.
This trimming can be performed as soon as the mold is closed, i.e. while the sheet is still hot, or after the sheet has been allowed to cool for a predetermined length of time.
Trimming is preferably performed by cutter means mounted on a cursor that is movable relative to the mold.
In a particular embodiment, the side wall of the cursor is arranged to co-operate with the mold so as to constitute a compression chamber enabling thermoplastic material to be overmolded onto the sheet.
The use of such a cursor makes it possible to trim the sheet and to form the compression chamber in a single operation, thereby further increasing production rates.
In a particular embodiment, the sheet is overmolded at least locally by extruding thermoplastic material prior to closing the mold.
In a variant, the sheet is overmolded by injecting thermoplastics material into the mold after it has been closed.
Still in a particular embodiment, the first portion of the mold has one or more grooves and the second portion of the mold has one or more ribs arranged to engage in said groove(s) so as to trim the sheets, at least in part, around the outline(s) of the groove(s) during closure of the mold.
With such a configuration it is possible to avoid using a cursor as defined above.
It is also possible to trim the part that is to be manufactured in part only, around a given outline, with said part remaining connected, after the mold has been opened, to the remainder of the sheet by mere bridges of material that can easily be broken or cut through.
In a particular implementation, the first mold portion constitutes a die having a setback in its top portion so that the resulting part is generally in the form of a channel section, the web of the channel having an indentation whose concave side faces in the opposite direction to the concave side of the channel.
This shape of die is particularly suited to making a motor vehicle bumper beam.
Advantageously, the second mold portion has one or more pushers arranged to engage in the above-mentioned setback.
The present invention also provides a mold for forming a sheet of reinforced plastics material, the mold comprising first and second portions that are movable relative to each other, and being characterized by the fact that it includes one or more pushers mounted on the second portion and movable relative thereto so as to press said sheet against the first portion at least locally, and at least one trimming means enabling the portion of the sheet that projects from the join plane to be trimmed off after the mold has been closed.
The first portion can include a setback and at least one of the pushers can advantageously present an end whose profile substantially matches the shape of the setback.
The pusher can thus contribute to compacting the sheet.
One of the two mold portions can include one or more grooves and the other mold portion can include one or more ribs arranged to engage in said groove(s) so as to trim the resulting part, in part.
The invention also provides a part made of reinforced plastics material obtained by implementing the method as defined above.